Regulated power sources are indispensable in modern electronics. For example, the power supply in a personal computer often needs to receive power input from various outlets. Desktop and laptop computers often have regulated power supplies on the motherboard to supply power to the CPU, memories, and periphery circuitry. Regulated power supplies are also used in a wide variety of consumer applications, such as home appliances, automobiles, and portable chargers for mobile electronic devices, etc.
Additionally, regulated power sources are also used in other applications. For example, a regulated voltage is often used to control the speed of a DC motor, e.g., for driving a cooling fan in a computer. As modern personal computers become more powerful, they consume more power and generate more heat. As a result, regulator circuits for fan speed control have become increasingly prevalent.
In general, a voltage or current source can be regulated using a linear regulator or a switching mode controller. A linear regulator maintains the desired output voltage by dissipating excess power. In contrast, a switching mode controller rapidly switches a power transistor on and off with a variable duty cycle or variable frequency and provides an average output that is the desired output voltage.
Switching mode power supplies (SMPS) have the advantages of smaller size, higher efficiency and larger output power capability, and are widely applied in mobile phone chargers, notebook computer adapters, and other fields. In recent years, green power supplies are emphasized, which require higher conversion efficiency and especially lower standby power consumption.
Pulse Width Modulation (PWM) control circuits have been widely used in switching mode power supplies. In PWM control circuits, the duty cycle of the control pulse is used to control the output of a device. In various applications, it is often desirable to set a minimum duty cycle of the output pulse, such that the duty cycle of the output pulse of the PWM control circuit does not become smaller than a predetermined value.
FIG. 1A illustrates the operation of a conventional PWM controller circuit having a minimum duty cycle feature. FIG. 1A shows a triangle wave signal 102, an input control signal 104, a minimum duty cycle setting voltage 106, and a PWM output signal 108. As shown, triangle wave signal 102 oscillates between an upper limit voltage VH and a lower limit voltage limit VL. The duty cycle of PWM output signal 108 is determined by comparing triangle wave signal 102 with a reference voltage. In an example, the PWM output signal is high when triangle wave signal is higher than the reference voltage, and PWM output signal is low when the triangle wave signal is lower than the reference voltage. In FIG. 1A, the reference voltage is the lower one of input control signal 104 and minimum duty cycle setting voltage 106, which intersect each other at crossover point 110. When input control signal 104 is lower than triangle wave signal 102, as shown in FIG. 1A to the left of crossover point 110, the duty cycle of PWM output signal 108 is controlled by input control signal 104. When input control signal 104 is higher than minimum duty cycle setting voltage 106, as shown in FIG. 1A to the right of crossover point 110, the minimum duty cycle feature is in activated, and the duty cycle of PWM output signal 108 is controlled by minimum duty cycle setting voltage 106.
Even though conventional PWM controllers are used in some applications, they suffer from many limitations. As discussed below, these limitations include insufficient power output at low power supply conditions and inefficient power utilization at high power supply conditions.
From the above, it is seen that improved PWM control circuits and methods are desired.